Solar water splitting and hydrogen generation is an essential step of artificial photosynthesis for the direct conversion of solar energy into chemical fuels. Among the various approaches, one-step photochemical water splitting is of particular interest because of its simplicity, low-cost operation, and its ability to use nearly neutral pH water, such as sea water, for large scale solar-fuel production. It is a “wireless” version of photoelectrochemical water splitting in which the counter electrode is mounted on the photocatalyst surface in the form of micro/nano-electrode i.e. co-catalyst. Accordingly, there is no requirement for an external bias, and hence no circuitry is required for its operation, and its efficiency is not limited by the low current conduction issue in the conventional Z-scheme process. Within the currently known photocatalysts the group III-nitride semiconductors, e.g. InGaN, represent the only material whose band gap energy can be tuned across nearly the entire solar spectrum as well as straddle the water redox potentials under ultraviolet (UV), visible, and near-infrared light irradiation.
Accordingly, InGaN promises high efficiency overall water splitting under one-step photo-excitation. The extreme chemical stability of metal-nitride further supports their use as an alternative photocatalyst. To date, however, the efficiency of overall water splitting using InGaN and other visible light responsive photocatalysts has remained extremely low, see for example Kibria et al in “One-Step Overall Water Splitting under Visible Light using Multiband InGaN/GaN Nanowire Heterostructures” (ACS Nano, Vol. 7, pp. 7886-7893) and Kubacka et al in “Advanced Nanoarchitectures for Solar Photocatalytic Applications” (Chem. Rev., Vol. 112, pp. 1555-1614). While much of the prior art research has focused on enhancing the optical absorption through band gap engineering, see for example Kubacka, the detrimental effects of unbalanced charge carrier extraction/collection on the efficiency of the four electron-hole water splitting reaction has remained largely unaddressed. Accordingly, it would be essential to address the reduced efficiency arising from unbalanced charge carrier extraction/collection, allowing increased efficiency of standalone photocatalytic hydrogen generation through solar powered water splitting.
Recently, nanoscale photocatalysts have been intensively studied which can increase light absorption and charge carrier separation, and therefore enhance the quantum efficiency, see for example Tong et al. in “Nano-Photocatalytic Materials: Possibilities and Challenges” (Adv. Mater., Vol. 24, pp. 229-251). Fermi-level pinning, however, has been commonly measured on nanowire surfaces such that the resulting surface band bending creates an additional energy barrier for charge carrier transport to the photocatalyst-water interface leading to significantly reduced reaction rate and extremely low efficiency. To date, the rational synthesis of nanostructured photocatalysts with controlled surface charge properties, i.e. tunable surface Fermi level and band bending, has remained a near-universal challenge, see for example Tong. Such uncontrolled surface charge properties can further contribute to the photo-corrosion and instability of various nanostructures under harsh photocatalysis conditions, severely limiting their practical applications.
Accordingly, it would be beneficial to address these limitations by providing controllable dopants during the growth process to adjust the properties of nanoscale photochemical water splitting devices in order to provide the appropriate Fermi level and/or band bending in order to allow the photochemical water splitting to proceed at high rate and high efficiency.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.